1. Field of the Invention
This invention relates generally to solar cell arrays and a method of making them. More particularly, the invention relates to an integrated array of thin-film, photovoltaic cells connected in series and/or parallel and a method of making it.
2. The Prior Art
Since each commonly known, individual solar cell generates only a small amount of power, usually much less power than is required for most applications, the desired voltage and current is realized by interconnecting a plurality of solar cells in a series and parallel matrix. This matrix is generally referred to as a solar cell array, and generates electrical energy from solar radiation for a variety of uses.
Solar cell arrays can be made manually by bonding the individual cells to a suitable support in the desired configuration and connecting, e.g., by soldering, the electrical leads of the individual cells in the necessary manner to give the desired voltage and current. Manual construction of arrays suffers from a number of disadvantages, including cumbersome and difficult construction methods, expense, faulty connections, and the like. An integrated array and method of making it is described in U.S. Pat. No. 3,483,038, issued Dec. 9, 1969 to Hui et al. The patented array comprises a substrate of flexible plastic insulating material such as polyimide plastic to which a plurality of serially connected individual cells are integrally united. The individual cells comprise a bottom electrode of a three-layered metal film covered by a film of an n-type semiconductor such as cadmium sulfide and a film of p-type semiconductor such as cuprous sulfide to form a barrier layer, and a top electrode of a thin film of metal such as tellurium.
The use of flexible plastic films of the prior art as substrates has certain disadvantages. Many of the plastics are air and water permeable to a certain degree which makes it impossible to completely hermetically seal the final cell array and can result in degradation of the cell components over a period of time as air and water diffuse into the cell. It is frequently difficult to bond the metal electrode to the plastic. The plastic substrate is too flexible for many uses and cell manufacturing techniques, so rigidity must be built in by later encapsulation, or the flexible substrate must be supported with a rigid structure. Bonding the plastic film to a rigid structure can produce degradation of the film by mechanical deforming of the film from bonding pressures, or from heat used in thermal bonding or from solvents used in adhesives. Many plastics have absorption peaks in the infared, resulting in high temperatures in use when these plastics are used in photovoltaic cell construction. Flexing of the substrate over a period of time of use tends to break down the films in the cells, reducing their output and shortening their life. The inherent flexibility of plastic substrate material increases processing complexity, since in order to obtain high resolution of array cells, the plastic film can not be subjected to stresses during the manufacturing process which would cause stretching and buckling. Use of a plastic substrate limits the temperature to which the cell can be subjected during fabrication.
Abrahamsohn in U.S. Pat. No. 3,376,163, issued Apr. 2, 1968, illustrates the use of commercially available conductive glass as a cell substrate. These are glasses that have a thin layer of conductive tin oxide on the surface. These glasses are typically produced by spraying a solution of tin chloride on hot glass, above 800.degree. F. See U.S. Pat. No. 2,648,753, issued Aug. 11, 1953, to Lytle for a description of the process. This process has several disadvantages. Because the tin salt solution is sprayed onto a hot glass surface, the conductive layer is continuous on the surface of the glass and it is not possible to obtain the complicated grid pattern needed to obtain a parallel or series array of cells on a simple substrate.